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排序方式: 共有539条查询结果,搜索用时 15 毫秒
531.
Andreas T. Messmer Katharina M. Lippert Sabrina Steinwand Dr. Eliza‐Beth W. Lerch Kira Hof David Ley Dr. Dennis Gerbig Dr. Heike Hausmann Prof. Dr. Peter R. Schreiner Prof. Dr. Jens Bredenbeck 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(47):14989-14995
Determining the structure of reactive intermediates is the key to understanding reaction mechanisms. To access these structures, a method combining structural sensitivity and high time resolution is required. Here ultrafast polarization‐dependent two‐dimensional infrared (P2D‐IR) spectroscopy is shown to be an excellent complement to commonly used methods such as one‐dimensional IR and multidimensional NMR spectroscopy for investigating intermediates. P2D‐IR spectroscopy allows structure determination by measuring the angles between vibrational transition dipole moments. The high time resolution makes P2D‐IR spectroscopy an attractive method for structure determination in the presence of fast exchange and for short‐lived intermediates. The ubiquity of vibrations in molecules ensures broad applicability of the method, particularly in cases in which NMR spectroscopy is challenging due to a low density of active nuclei. Here we illustrate the strengths of P2D‐IR by determining the conformation of a Diels–Alder dienophile that carries the Evans auxiliary and its conformational change induced by the complexation with the Lewis acid SnCl4, which is a catalyst for stereoselective Diels–Alder reactions. We show that P2D‐IR in combination with DFT computations can discriminate between the various conformers of the free dienophile N‐crotonyloxazolidinone that have been debated before, proving antiperiplanar orientation of the carbonyl groups and s‐cis conformation of the crotonyl moiety. P2D‐IR unequivocally identifies the coordination and conformation in the catalyst–substrate complex with SnCl4, even in the presence of exchange that is fast on the NMR time scale. It resolves a chelate with the carbonyl orientation flipped to synperiplanar and s‐cis crotonyl configuration as the main species. This work sets the stage for future studies of other catalyst–substrate complexes and intermediates using a combination of P2D‐IR spectroscopy and DFT computations. 相似文献
532.
Winkelmann M Schneider L Gerlinger W Sachweh B Miller R Schuchmann HP 《Journal of colloid and interface science》2012,369(1):164-169
Rhenium (Re) nanoparticles have been synthesized by pulsed-laser decomposition of ammonium perrhenate (NH(4)ReO(4)) or dirhenium decacarbonyl (Re(2)(CO)(10)) in the presence of 3-mercaptopropionic acid (MPA) as capping agent, in both aqueous and organic media. Preliminary studies showed that the MPA-capped Re nanoparticles are capable of catalyzing the isomerization of 10-undecen-1-ol to internal alkenols via long chain migration of the C=C double bond at ca. 200°C. A one-pot synthesis of graphite-coated Re nanoparticles has also been achieved by pulsed-laser decomposition of Re(2)(CO)(10), due to photo-induced catalytic graphitization of the phenyl groups of PPh(3) on the surface of rhenium nanoparticles. 相似文献
533.
Matthias Schwalbe Heike Schreer Jens Langer Helmar Görls 《Journal of organometallic chemistry》2006,691(23):4868-4873
The nickel(0) complex [Ni(bpy)(cod)] (bpy: 2,2′-bipyridine, cod: cycloocta-1,5-diene) was used as a mild reducing reagent for the synthesis of the extremely reactive low-valent palladium complexes [Pd2X2(cod)2] (1: X = Cl, 2: X = Br), Pd(cod)2 (3) and Pd(norbornene)3 (4). The X-ray analysis of 1 showed that the two [Pd(cod)(Cl)] moieties are only connected by a short Pd(I)-Pd(I) bond (bond length: 2.5379(4) Å) with the chloride ions as monodentate ligands. The X-ray structure of 3 which is also known to be an extremely reactive compound could be determined by X-ray diffraction. As expected, the Pd(0) centre is surrounded by the two cod ligands to form a PdC4 tetrahedron with typical Pd-C bond lengths. The crystal structure of 3 shows it to be very similar to the closely related complexes M(cod)2 (M: Ni, Pt). The X-ray structure of 4 displays that the Pd(0) centre is in a trigonal planar environment of the three olefin groups. According to 1H NMR measurements the complexes have the same structure in solution as found in the solid state. 相似文献
534.
Schreiner PR Fokina NA Tkachenko BA Hausmann H Serafin M Dahl JE Liu S Carlson RM Fokin AA 《The Journal of organic chemistry》2006,71(18):6709-6720
The selective functionalizations of the fundamental hydrogen-terminated nanodiamonds triamantane 1, as well as the most symmetrical representative of the tetramantanes (C(2h)-[121]tetramantane 2) were elaborated. Electrophilic reagents (Br2, HNO3) predominantly attack the medial C-H positions of the cages; bromination of 2 gave the medial 2-bromo derivative almost exclusively. Highly selective apical substitution in 1 and 2 is possible either under single-electron-transfer oxidations via hydrocarbon radical cations or through photoacetylation with diacetyl. The mono- and the bis-acetyl derivatives of 1 and 2 were converted through Bayer-Villiger oxidation and subsequent hydrolysis to the respective apical mono- and dihydroxy derivatives. This exceptional synthetic specificity facilitates the transformation of 2, and perhaps larger nanodiamond molecules, into functionalized building blocks needed for a wide range of applications such as nanotechnology. 相似文献
535.
Nikola Getoff Heike Schittl Johannes Hartmann Ruth Maria Quint 《Journal of photochemistry and photobiology. B, Biology》2009,94(3):179-182
Testosterone (TES; 4-androstene-17β-ol-3-on) is found for the first time to eject electrons from its singlet excited state in water–ethanol solvent mixture. This ability was very recently also observed for 17β-estradiol (17βE2) and progesterone (PRG)/1/. With increasing TES-concentration, the yield of solvated electrons () is decreasing, because of “associate” formation. At higher absorbed UV-doses (λ = 254 nm) the yield is passing a sharp maximum by formation of TES–ethanol adducts, which are able likewise to emit electrons when excited. At prolonged irradiation the resulting photolytic products of TES–ethanol adducts are also able to emit electrons.The capability of the hormones: 17βE2, PRG and TES to eject electrons and the resulting metabolites, some of which can induce cancer, is discussed. 相似文献
536.
Hadley Wickham Michael Lawrence Dianne Cook Andreas Buja Heike Hofmann Deborah F. Swayne 《Computational Statistics》2009,24(2):207-215
What is a pipeline, and why do we need one for interactive graphics? This conceptual paper attempts to answer these questions, building on previous work. A pipeline controls the transformation from data to graphical objects on our screens, and we argue that the pipeline must be present, in some form, in all graphics software. The pipeline is made explicit in descendants of DataViewer. 相似文献
537.
Heike Traub Markus W?lle Joachim Koch Ulrich Panne Ralf Matschat Heinrich Kipphardt Detlef Günther 《Analytical and bioanalytical chemistry》2009,395(5):1471-1480
Solution-doped metal powder pellets as well as aspirated liquids were used as calibration samples to analyze pure copper and
zinc certified reference materials (CRMs) by femtosecond laser ablation ICP-MS. It was demonstrated that calibration by copper
pellets resulted in relative deviations up to 20%, whereas fs-LA-ICP-MS among copper-based CRMs led to inaccuracies in the
same range unless nominal mass fractions were chosen to be <3 mg/kg. Calibration by zinc pellets generally provided better
accuracy. Depending on the analyte considered, deviations below 10% were obtained even for mass fractions close to the limit
of quantification. Our data, therefore, indicate solution-doped metal powder pellets to be suitable as calibration samples
for fs-LA-ICP-MS of metals. Furthermore, the utilization of liquid standards for calibration was found to result in stronger
deviations of up to 50% for both copper and zinc samples which, in addition, turned out to be dependent on the plasma conditions.
相似文献
538.
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